1. Field of the Invention
The present invention is directed to a radiation converter for converting x-rays into electrical signals, from which a visible image of the x-ray image can be produced.
2. Description of the Prior Art
German OS 33 32 648 discloses a radiation converter which is fashioned as an image intensifier. Such image intensifiers include an input screen having a radiation absorber for generating light photons dependent on the radiation intensity of incoming radiation. The radiation absorber is followed by a photocathode, which generates electrons dependent on the light photons originating from the radiation absorber. These electrons are accelerated by an electrode system onto an electron receiver. In an image intensifier, this electron receiver is fashioned as an output screen that generates light photons due to the incident electrons.
In contrast to a non-destructive material inspection, the radiation load must be kept as low as is technically expedient when a patient is medically examined, so that the radiation load on the patient is as low as possible. To achieve this goal, efficient utilization of the radiation that penetrates the patient and strikes the radiation receiver is of paramount importance. However, the lower the intensity of the radiation incident on the radiation receiver, the lower are the signals that can be derived from the radiation receiver. The amplitude difference between useful signal levels and noise signals also becomes less, which degrades the diagnostic content of the image generated by means of these signals. Therefore, a compromise must be made between a low radiation load for the patient and a radiation dose that is strong enough for allowing a good diagnosis from radiation images of the patient.
Photographic film functions merely as a chemical intensifier, which intensifies the ionization processes of the radiation in the microscopic domain by many dimensions and thus makes the ionizing effects visible in the macroscopic domain.
Storage luminophore plates latently store the radiation shadow image of a subject. On the basis of the latent image, light photons are generated by scanning the storage luminophore plate with a light beam. These light photons are converted into electrons by a readout system with a photomultiplier, whereby the electrons, almost without noise, can be intensified up to a factor of 106 and can be converted into electrical signals. Then, these electrical signals are available for representing the image.
The geometric reduction, which results from a large input window and a small output window, is used with respect to X-ray image intensifiers for intensifying the luminance, guided by the xe2x80x9cextraxe2x80x9d energy absorbed of the electrons propagating from the input fluorescent screen to the output fluorescent screen through an accelerating field therebetween.
In detectors referred to as flat panel image detectors, a layer which transforms radiation into light and which, for example, contains Csl, is brought in contact with a photodiode matrix composed of amorphous silicon, so that the light photons generated by the layer due to incident radiation can be converted via the photodiode matrix into electrical signals, which then can be utilized for the image representation. Since the light photons are not intensified via the electrons, only relatively weak signals can be obtained from the photodiode matrix, which can only be intensified in a following device, such as an intensifier. Since the charge packets of these relatively weak electrical signals then must also be guided to the intensifier via complicated clocking methods from the large-area flat image detectors via relatively long lines, the average noise, measured in electrons, is almost twice as much as the signal generated by individual X-ray quanta. Particularly for fluoroscopy, wherein only low X-ray doses are applied, the signals that can be obtained from the flat panel image detector are particularly low and are situated close to the noise range and therefore require complicated procedures for artefact correction. In fluoroscopy, the signals of every other scanning ray are inspected (analyzed) for correction purposes, so that the conventional image repetition rates are far from being able to be achieved. The dynamic range of the signals that are obtainable from the flat panel image detector is also considerably restricted.
An object of the present invention is to provide a radiation converter of the type described above wherein signals, by means of which image signals that still can be appropriately diagnosed can be generated in a following signal processing chain at a display, can be derived at the output of the radiation converter even when the radiation intensity is low.
The object is inventively achieved in a radiation converter having an electron multiplier between an electron detector, which is fashioned as an electron receiver, and the photocathode, the electrons originating from the photo cathode being multiplied via the electron multiplier. Thus, a multiplication of the electrons issuing from the photocathode, and therefore a signal boost of the signals that can be obtained from the electron detector, occur, so that relatively high signals can be obtained at the electron detector even when the intensity of the radiation that is incident on the radiation absorber is relatively low.
It is advantageous to provide a common gas-proof housing for the electrode system, the electron multiplier and the electron detector, so that a compact structure of the radiation converter results. Preferably, the housing contains gas having at least one of the following constituents: argon, krypton, xenon, helium, neon, CO2, N2, hydrocarbon, Di-methyl-ether, methanol-vapor, ethanol vapor. (As used herein, the term xe2x80x9cgasxe2x80x9d encompasses xe2x80x9cgas mixture.xe2x80x9d) As a result of the admixture of the aforementioned elements and/or compounds, UV light photons are absorbed and do not reach the photocathode, where they would disadvantageously contribute to the generation of electrons.
The radiation absorber particularly transforms radiation into light photons in an advantageous manner when it has a needle-shaped structure and is composed of Csl:Na.
If the intensification of the electrons is to be further increased, it is advantageous to employ a number of electron multipliers each of which can be fashioned as a wire grid, for example. According to a particularly advantageous embodiment, an apertured plastic film that is provided with a metallization on both sides can be provided. Expediently, the plastic film is made of polyimide and the metallization of copper. It is also expedient when the holes of at least two of the electron multipliers are offset relative to one another, so that an increased number of electrons and a beneficial construction of the electron multiplier result, and so that a backscattering of UV-photons onto the photocathode is avoided.
When the photocathode is fashioned of nonconducting or essentially nonconducting material, it is advantageous to provide an electrically conducting intermediate layer between the ray absorber and the photocathode as an electrode, which is preferably composed of gold, so that electrodes can be made available to the photo cathode in this way and so that it is not electrically charged during the operation.
It is particularly advantageous when the electron detector is fashioned as a 2D thin-film panel and is composed of a-Se, a-Si or poly-Si. Such an electron detector has a simple structure and is cost-efficient.